The generation of light from electroluminescent solid state light emitting devices (EL devices), as described in this application, is based on applying energy from an electric field to a light emitting structure including an active region or emissive layer. The emissive layer may comprise a wide band gap semiconductor or dielectric e.g. silicon nitride, silicon dioxide, or GaN, which may include luminescent centres, such as nanocrystals and/or rare earth species. It is important to deliver a minimum and controlled amount of energy to luminescent centres in an active light emitting layer in the device. If the energy of incident electrons is too low there will be no light emission possible. On the other hand, if the electrons possess too much energy there will be light emission but excess energy will be carried away in the form of heat, which reduces efficiency. Furthermore, hot electrons can be responsible for damage to the host matrix, result in charging, and ultimately contribute to breakdown and failure of the device under bias.
For example, as described in pending United States Patent Publication No. 2008/0093608 filed Dec. 12, 2006, entitled “Engineered structure for solid state light emitters”, a device is described comprising a light emitting structure having a plurality of emissive layers, i.e. optically active layers comprising luminescent centres, that are separated by drift layers. The light emitting structure is disposed between electrodes for applying an electric field to the light emitting structure. One electrode, usually a top electrode, comprises a transparent conductive oxide (TCO), typically a layer of indium tin oxide (ITO) or other suitable transparent conductive material, which not only provides for electron injection, but also allows light to be extracted from the EL device. Electrons are accelerated and gain energy from an applied electric field as they traverse each drift layer, and energy is released as light from luminescent centres in the active layer by impact ionisation or impact excitation. The drift layers may alternatively be referred to as drift regions, buffer layers or acceleration layers.
In the ballistic regime, the kinetic energy in electron volts gained by an electron passing through the drift region is E×d where E is the electric field across a drift layer in V/cm and d is the thickness of the drift layer in cm. Thus, as described in United States Patent Publication No. 2008/0093608, by selecting the appropriate thickness of drift layer, matched to an excitation energy of the active layer, electrons gain the necessary energy to excite the emissive layer. Careful consideration and design of the drift layer in conjunction with the operating electric field allows tuning of the electron energy with the drift layer thickness. The drift layers may be made from a wide bandgap semiconductor or dielectric material, such as oxides or nitrides of silicon. The structure may be deposited by techniques such as CVD (chemical vapour deposition), PECVD (plasma enhanced CVD), sputtering, ALE (atomic layer epitaxy) and MBE (molecular beam epitaxy).
The TCO electrode has a critical role in the performance of an electroluminescent device of this type. Indium tin oxide (ITO) is the most commonly used TCO. There are many other known TCOs, e.g. binary and ternary compounds such as ZnO and SnO2, which are being explored as an alternative to ITO for EL devices. Some other binary compounds of particular interest as alternative to ITO are aluminum doped ZnO (AZO), indium doped ZnO (IZO), and TiO2 [anatase] doped with Ta or Nb. However, use of these compounds is currently less established in the industry, and it has yet to be demonstrated that they can match the electrical properties of ITO.
Although TCOs, such as ITO, have been found to tolerate high currents at low fields, TCOs in EL devices have a tendency to fail when they are simultaneously exposed to large electric fields and high electron fluxes. These conditions may be encountered, for example, in operating devices under conditions to maximize light emission for application to solid state lighting, where high brightness is required.
As an example, during testing of a EL device structure in which the emissive layer comprises a silicon rich silicon oxide layer containing silicon nanoparticles, and the drift layer comprises silicon dioxide, the structure was found to suffer from charging effects that resulted in the device becoming unstable and ultimately led to breakdown of the device under bias. It is believed that hot electrons are responsible for damage and for charging effects observed in the structure when biased at a constant current density.
While observing the spectrum of emission from a device comprising an ITO top electrode during failure, as shown in FIG. 1, two very bright lines appeared at 452 nm and 410 nm. These lines have been identified as originating from singly ionized Sn and In respectively (NIST Atomic Spectra Database). This suggests that the ITO is dissociating as the bias, electric field and electron flux are increased, and the damage threshold of the ITO is exceeded. In fact, He et al (“Damage study of ITO under high electric field”, Thin Solid Films, 363 (2000) pp. 240-243) have shown that exposure of ITO to large electric fields greater than 1 MV/cm can result in decomposition of ITO and the physical migration of In and Sn within the film. Energies associated with the In peak suggest that there are electrons with energies greater than 3 eV impinging on the ITO contact layer. Similar spectra have been observed from AZO failure in similar devices under bias.
One way to reduce the probability of hot electrons passing unobstructed through the emissive layer is to ensure the density of optical centres is high enough that the electron capture cross section of the luminescent centres in the active layer makes the layer effectively opaque to incident electrons. The electron capture cross section depends on the particular optical/emissive centre(s) used, and thus the required density, or concentration, of optical centres is dependent on the species and its capture cross section. When the optical centres are rare earth ions, such as terbium, it is known that higher concentrations (e.g. densities of greater than ˜4% for Tb, or less for other rare earth species) give rise to quenching of optical centres due to cross relaxation and clustering effects (J. Sun et al., J. Appl. Phys. 97, 123513 (2005)). Thus there may be practical limits to increasing the density of optical centres in thin layers. Another alternative is to increase the thickness of the emissive layers to make the layer sufficiently opaque to electrons. However, this solution would require additional voltage to bias and support the electric field in these regions. This would have a negative effect on the efficiency of such a device as the applied voltage is directly related to the input power, and may be undesirable for other reasons.
Copending United States Patent Publication No. 2007/0181898 and United States Patent Publication No. 2008/0246046 disclose the use of a thin, conductive transition layer between the emitter structure and the top and bottom contact electrodes, e.g. to control current injection. As described, this layer may also help to reduce hot electron effects and provide shielding to the current injection layer. A conducting layer (metallic or silicon-rich silicon oxide) must be thin, because it is generally opaque to the transmission of light. Thus, a thin conductive layer has only limited ability to shield the TCO layer from hot electrons, without impeding light extraction.
There is, therefore, a need for alternative solutions to reduce deleterious hot electron effects, such as charging, damage, or other effects leading to breakdown, and/or decomposition of TCO electrode materials, or similar hot electron effects within the emitter layer structure.
Consequently, the present invention seeks to overcome or mitigate the above mentioned problems, or at least provide an alternative.